/Engine/Source/Editor/UnrealEd/Public/DragTool_Measure.h
Starts a mouse drag behavior. The start location is snapped to the editor constraints if bUseSnapping is true.
virtual void StartDrag ( [FEditorViewportClient](API\Editor\UnrealEd\FEditorViewportClient) * InViewportClient, const FVector & InStart, const FVector2D & InStartScreen )